Conventionally, the cores which are used in the construction of vehicle seats, bottoms and the backs and the partitions which are used to divide the passenger occupied space from that of luggage and/or tire space are made of metal or synthetic structures. Metal structures were used to provide strength; however, their weight and cost of manufacture are excessive. Synthetic structures were of two types, pressure-molded and blow-molded, both lighter in weight and less expensive to manufacture than metal structures, but lacking the strength of metal structures. Of the two types, blow-molding is the more desirable since it is more economical of material and because the hollow construction provides for maximum weight reduction. A hollow blow-molded structure of the kind referred to is shown in FIGS. 1, 2 and 3 and it will be seen therein comprises front, back and side walls 11, 12 and 13. To improve the strength of such hollow structures, front and/or back or both front and back walls are provided with recessed ribs 14 and 16 positioned longitudinally and transversely of the structure. As shown in FIG. 2, the ribs 14 and 16 are formed in the front wall and are attached at their apices 15 internally of the structure to the back wall. In FIG. 3, wherein the ribs are formed in both front and back walls, the apices 15 of the ribs are attached to each other within the hollow structure. The structure shown in FIGS. 1, 2 and 3 are of improved stiffness over that of a structure lacking such ribs; however, because of the concavity of the ribs, the latter form potential zones of bending parallel to their length such that when the bending stresses reach a predetermined level, the structure tends to flex at the apices of the ribs. Further, the concave ribs present an undesirable surface effacement which, preferably, should be totally planar.